I would start by teaching her the names of the letters and their sounds. This is the letter 'A', 'A' says a, a, a,. Then you could show her some 'A' words and pictures will help. Ant, Apple etc. She needs to be able to recognise the letters & know the sounds first. Maybe get some flash cards with bright pictures and simple words on them.
You could also have a list of golden words that you could begin practicing together such as 'at, is, in, I, me' etc. I'm sure if you googled them they would come up.
3 isn't too early especially if she wants to learn. Good luck